Code 41 for CM or KOER? Knowing that helps get an idea when the problem occurs but sounds like all "CM" based on what you've stated so far.

And yea I agree best bet start with fuel pressure.

4 volts might be nothing more than end result of "margin of error" in the meter you're using, margin of error +/- represents accuracy percentage and near full selected scale. Further away from that you get margin of error increases and reason always want to use lowest range possible for test.

02 sensor operatates between 0 and about 1 volt so even at 4 volts reference its more than enough or should be. 0v = lean 1 volt = rich.

Might be worth a check with a different meter making sure to use lowest scale offered above 5volts (equal scale to first meter first), I would if for no other reason to satisfy my curiosity about reading received.

If meter set to 20volts (0 - 20volts lowest DC range tends be the norm for basic multi meters) and was found to be off by a full volt when testing down at/just under 5 volts? Yea I'd probably pitch it in the trash can.

Run KOER with fuel pressure gauge connected, see if get code 41 if all previous code 41 had been CM and KOER test hasn't been done yet.

If has been a CM code make sure clear codes and then if at all possible put the fuel pressure gauge where can be seen than take it for a drive test see if the pressure drops, thats if its within spec sitting still.